We synthesized Cu modified S-doped g-C₃N₄ thin sheets using calcination followed by a wet-impregnation method. The photocatalytic activity was studied for reduction of CO₂ to CO and CH₄ in the presence of water and a plausible mechanism is explained.

Strategic doping of metals and non-metals into a photocatalyst can help in tuning the band gap and alignment of band structure. The surface charge and charge transfer mechanism in a photocatalyst system govern the amount of products and their distribution. Herein, we used a wet impregnation method to embed Fe and Cr into the in-planes of porous boron-doped carbon nitride and tested the photoactivity by reducing CO₂ gas in the presence of water at room temperature under visible-light illumination. The total consumed electron number, a measure for photoactivity, was increased by 2.14 times for Cr,B-codoped carbon nitride than that of the pristine graphitic carbon nitride. Also, the selectivity for CO was increased with transition metal doping. X-ray photoelectron spectroscopy revealed that Fe existed in the +2 and +3 oxidation states; while Cr was in the +3 and +6 states. Notably, the optical and electronic properties were altered for the transition metal-doped catalysts. Several in situ measurements were performed to identify the reaction intermediates and underlying mechanism. COO⁻ and HCHO were active intermediates for CO formation, and HCO₃ ⁻ could be an intermediate for CH₄ generation. This work elucidates the pivotal role of trapped charge carriers in enhancing charge separation efficiencies and product selectivity. The results herein clearly demonstrate a promising strategy to use the strong p–d repulsion between metal and non-metal elements on a host photocatalyst to extend the visible-light absorption range and reduce the recombination events, which can enhance the selectivity and yields of products.